A number of medical devices such as, e.g., hard shell venous reservoirs used in open heart surgery are shipped with plastic tubing which is set into a tight coil for shipment but is pulled into a generally straight configuration (much like the cord on a telephone handset) in use. Typically, the tubing coil is seated in a seating trench in the lid of the reservoir. The coil has a connector on each of its ends, one of which is permanently connected during assembly to a manifold formed on the lid. The other connector is loose, and is connected to other equipment by the user.
The coil is conventionally formed by winding it around a mandrel, and then giving it a permanent set in that form by the application of heat or solvents such as methylethylketone or toluene. Either process not only sets the tubing to its coiled shape but also causes the convolutions of the coil to adhere to each other. This is necessary in order for the coiled tubing to fit exactly into the seating trench, and to resist any axial expansion of the coil during assembly and shipping which would cause it to pop out of the trench. Also, the adhesion of the convolutions allows the end user to pull out only enough convolutions to reach the equipment to which the tubing is to be connected, thereby preventing the tubing from hanging down where it can become entangled or stepped on.
Several problems arise in the fabrication of these tubing coils. One is that the convolutions must stick together enough so that they will not come apart in assembly or shipping, but not so much that they will offer any significant resistance or suffer damage when they are pulled apart by the end user. Both the heating method and the solvent method of setting the coils are deficient in that respect because in both processes, the degree of adhesion of the convolutions is not readily controllable.
A more serious problem arises from the fact that medical equipment is typically manufactured and assembled in a cleanroom. The use of chemical solvents in a cleanroom is highly undesirable because they are usually toxic, corrosive and flammable. The presence of heating elements in the cleanroom is also undesirable because of the presence of isopropyl alcohol fumes which ignite easily due to their low flash point. Heating elements also pose the additional risk of damaging the plastic tubing by melting or burning it if operating parameters are not strictly observed. These factors commonly result in the need to rework 5-10% of the coils in a production setting.